This invention is generally in the field of reference electrodes and in particular the area of electrodes for use as sensors for monitoring and controlling the process of electrolytic reduction of aluminum.
All primary aluminum extraction is done by electrolysis. In the Hall-Heroult process which predominates in the industry, aluminum oxide is dissolved in molten cryolite, Na.sub.3 AlF.sub.6, which serves as the electrolyte. Aluminum oxide is decomposed into molten aluminum and carbon dioxide gas by the passage of electric current through the melt. A second electrolytic process involves the electrolysis of aluminum chloride dissolved in a solution of alkali chlorides, typically sodium chloride and lithium chloride. Aluminum chloride is decomposed into molten aluminum and chlorine gas by the passage of electric current through the melt.
Control of the electrochemical process requires the use of a reference electrode, a device in which a constant electrical potential (voltage) is established through the maintenance of thermodynamic equilibrium between chemically reacting species. A reference electrode can be thought of as an electrochemical probe or sensor. The reference electrode facilitates measurement of the concentration of aluminum in the bath and overvoltages at either the anode or cathode.
Prior art aluminum reference electrodes have not been able to provide a stable, drift-free output voltage. Prior art reference electrodes include gas electrodes; oxide electrodes (Fe.sub.2 O.sub.3 ; Fe.sub.3 O.sub.4 ; Cr.sub.2 O.sub.3 ; SnO.sub.2); metal electrodes (aluminum; Fe-Al, Pt-Al, or Pb-Na alloys; Pt, W); and liquid junction electrodes.
As described by K. Grjotheim et al. in Aluminium Electrolysis, 2nd ed., 195-206 (Aluminum-Verlag Dusseldorf FRG 1982), gas electrodes have problems with wear and lack of stability. Oxide electrodes are also unstable, possibly due to corrosion or the semiconducting nature of the oxides. Liquid junction electrodes require connection of the two half-cells by a diaphragm or salt bridge. Difficulties in finding a suitable diaphragm or salt bridge have restricted their use in cryolite melts. Further, mixing of the two melts continues to be a problem regardless of the material used to separate the half-cells.
An electrode comprising a molten salt and aluminum was first introduced by P. Drossbach in Z. Elektrochem., Vol. 42, 65 (1936). A common type consists of a thin-walled tube of sintered alumina or boron nitride (BN) containing a molten aluminum pool covered with molten salt (cryolite). Another type provides access to the electrode through a small hole in the tube. Electrical contact with the electrode is by means of a platinum, tantalum or tungsten wire. As with the other prior art reference electrodes, reaction between the wire and the salt causes instability. Attempts to make electrical contact with the molten aluminum without also making contact with the molten salt have failed. Since the salt melt is not only highly corrosive but has great capillarity, the choice of materials for insulating the lead wire to the aluminum pool is limited. The salt melt eventually penetrates the lead insulator and shorts the lead wire to the salt. After shorting occurs, a mixed potential is established, and the voltage of the reference electrode becomes unstable.
It is therefore an object of the present invention to provide a reference electrode that generates a stable, drift-free output voltage and is thus useful for process control of aluminum extraction.
It is another object of the invention to provide a reference electrode useful in highly corrosive melts for advanced electrochemical analysis techniques, such as cyclic voltammetry.
It is a further object of the invention to provide an aluminum reference electrode that is useful in the analysis of refractory solids.
It is a still further object of the invention to provide a reference electrode useful in the conduction of corrosion tests in molten fluorides.